add tests solidifying behavior for modelId with polymorphic models
add tests solidifying behavior for modelId with polymorphic models

ppker/backbone,moimikey/backbone,jridgewell/backbone,moimikey/backbone,darrrk/backbone,craigmichaelmartin/backbone,jashkenas/backbone,ppker/backbone,zgl2007dj/backbone,akre54/backbone,craigmichaelmartin/backbone,darrrk/backbone,jashkenas/backbone,lastjune/backbone,akre54/backbone,jridgewell/backbone,platinumazure/backbone,platinumazure/backbone,lastjune/backbone,zgl2007dj/backbone